# SMTP

SMTP ports you need to understand are:

* TCP 25: default for non-encrypted communications.
* TCP 465: port registered by IANA for SMTP over SSL (SMTPS). Note, this has been deprecated in favour of STARTTLS.
* TCP 587: Secure SMTP (SSMTP) for encrypted communication using STARTTLS.
* TCP 110: default port for POP3 non-encrypted communications.
* TCP 995: default port for POP3 encrypted communications.
* TCP 143: default port for IMAP in non-encrypted communications.
* TCP 993: default port for IMAP in SSL/TLS communications.

Some useful SMTP commands for validation are:

* HELO: initiate SMTP conversation with mail server.
* EHLO: initiate ESMTP conversation with mail server.
* STARTTLS: start a TLS connection to mail server.
* RCPT: denote mail address of recipient.
* DATA: initiate transfer of contents of an email message.
* RSET: cancel an email transaction.
* MAIL: used to denote mail address of sender.
* QUIT: used to close connection.
* HELP: display help.
* AUTH: authenticate client to server.
* VRFY: used to verify if a user's mailbox exists.
* EXPN: request, or expand a mailing list.